1. Field of the Invention
The present invention relates to an apparatus for controlling speed changes or shifts to be made by an automotive automatic transmission.
2. Description of the Prior Art
Some automatic transmissions for use on automobiles have a plurality of power transmitting gear trains which are selectively switched into operation to effect a speed change or shift by controlling engagement of engaging elements including clutches and brakes. The engagement of engaging elements is automatically controlled by hydraulic actuators to bring about automatic speed changes.
To effect a speed change in an automatic transmission, an engaging element that has been engaged, i.e., a previous-position engaging element, is disengaged, and an engaging element that is to be engaged to achieve a next gear position, i.e., a next-position engaging element, is engaged.
If the previous-position engaging element is disengaged too early, or the next-position engaging element is engaged too late, then there is a moment where any engaging elements are not engaged, causing the engine coupled to the automatic transmission to race. Conversely, if the previous-position engaging element is disengaged too late, or the next-position engaging element is engaged too early, then both the previous- and next-position engaging elements are engaged, resulting in a large speed change or shift shock.
There have heretofore been proposed various control systems for adjusting the timing of disengagement of the previous-position engaging element and engagement of the next-position engaging element to avoid the undesirable engine racing and shift shock.
For example, it has been customary to adjust the timing of engagement of an engaging element with an orifice positioned in a hydraulic passage which supplies working oil pressure to the engaging element, or with a valve which operates depending on the difference between working oil pressures applied to previous- and next-position engaging elements to forcibly lower the working oil pressure applied to the previous-position engaging element when the difference is equal to or higher than a predetermined value.
The timing adjustment with the orifice or the valve depends on the diameter of the orifice, the pressure-bearing area of the valve, and the load of a spring. Therefore, the timing of engagement of an engaging element tends to vary due to manufacturing errors of individual orifices and valves and aging of these orifices and valves.
Japanese laid-open patent publication No. 62-246653 discloses an apparatus for controlling speed changes in an automatic transmission. According to the disclosed apparatus, when a speed change is to be effected, the force applied to engage a previous-position engaging element is controlled in order to equalize the rotational speed of the input shaft of the transmission with a target rotational speed that is a predetermined value higher than the rotational speed of the input shaft immediately prior to the issuance of a command for the speed change, and at the same time the engagement of a next-position engaging element is controlled. When the speed change is effectively started by the engagement of the next-position engaging element, the force applied to engage the next-position engaging element is controlled in order to equalize the rate of change of the rotational speed of the input shaft with a target rate of change.
U.S. Pat. Nos. 4,938,102, 4,947,329, and 4,951,200 disclose speed change control processes for controlling the engagement of engaging elements based on the rotational speed of an engine and the rotational speed of the output shaft of a torque converter.
The disclosed apparatus and processes are free of variations of the timing of engagement of an engaging element due to manufacturing errors and aging of individual parts because the engagement of an engaging element is controlled based on the rotational speed of the engine, the rotational speed of the input shaft of the transmission, and the rotational speed of the output shaft of the torque converter.
According to a speed change control process disclosed in Japanese laid-open patent publication No. 1-261559, the period of operation of an automatic transmission for effecting a speed change is divided into a torque phase and an inertia phase, and feedback and feed-forward control processes are carried out in each of the torque phase and the inertia phase. A transition from the torque phase to the inertia phase is controlled based on a change in the output torque of the transmission.
Japanese laid-open patent publication No. 2-3770 shows another speed change control process in which for a downshift from a third gear position to a second gear position, the torque to engage an engaging means for achieving the third gear position is progressively reduced. When the rate of slippage of the engaging means exceeds a first reference value, the working oil pressure applied to the engaging means is maintained as it is, and when the rate of slippage of an engaging means for achieving the second gear position drops lower than a reference value, this engaging means is engaged thereby effecting the downshift.
In the above speed change control apparatus and processes, for adequate speed change control for each of different speed changes, the start of an effective speed change initiated by the engagement of a next-position engaging element is detected by determining whether or not the rotational speed of the input shaft of the transmission that has been equalized with the target rotational speed by controlling the engagement of a previous-position engaging element has dropped to the rotational speed of the input shaft immediately before the command for the speed change is issued. Theoretically, a smooth speed change is performed if the previous-position engaging element is disengaged and the next-position engaging element starts to be engaged when the rotational speed of the input shaft is equalized with the rotational speed of the input shaft immediately prior to the issuance of the command for the speed change.
Actually, however, a certain time delay or lag is unavoidable from the issuance of the command for a speed change to an actual change in oil pressure which causes the engaging element to operate. Therefore, the control process for disengaging the previous-position engaging element and engaging next-position engaging element is delayed if it is effected when the rotational speed of the input shaft of the transmission has dropped to the rotational speed of the input shaft immediately before the command for the speed change is issued. Consequently, the disclosed speed change control apparatus and processes suffer delayed disengagement of a previous-position engaging element and abrupt engagement of a next-position engaging element, thus impairing the feeling that the driver has upon speed changes.
Furthermore, as described above, when a speed change command is issued, the force applied to engage a previous-position engaging element is controlled in order to equalize the rotational speed of the input shaft of the transmission with a target rotational speed that is a predetermined value higher than the rotational speed of the input shaft immediately prior to the issuance of the speed change command, and when the speed change is effectively started by the engagement of a next-position engaging element, the force applied to engage the next-position engaging element is controlled. This control procedure is suitable for an upshift to be carried out while the engine is accelerated (power-on) by the depression of an accelerator pedal, but cannot be lied upon for a kickdown, i.e., a downshift to be carried out while the engine is accelerated.
After an upshift, the rotational speed of the input shaft of the transmission is lowered as long as the speed of the automobile remains the same. During the upshift, when the previous-position engaging element is disengaged, the rotational speed of the input shaft driven by the engine tends to rise because the engine is accelerated. To prevent the rotational speed of the input shaft from increasing away from the rotational speed thereof after the upshift, it is necessary to control the engagement of the previous-position engaging element to keep the rotational speed of the input shaft at a predetermined speed as described above.
After a kickdown, however, the rotational speed of the input shaft of the -transmission increases as long as the speed of the automobile remains the same. During the kick-down, it is preferable for a smooth and delay-free speed change to increase the rotational speed of the input shaft, rather than keeping it at the predetermined speed, and to engage the next-position engaging element when the rotational speed of the input shaft has increased up to a rotational speed corresponding to the speed reduction ratio at the next gear position.